1. Field of the Invention
This invention relates to a process for electrochemically producing chloric acid--alkali metal chlorate solutions. More particularly, this invention relates to the electrochemical production of chloric acid--alkali metal chlorate solutions suitable for the generation of chlorine dioxide.
2. Brief Description of the Prior Art
Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses due to its high oxidizing power. There is a variety of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators employed, for example, in pulp and paper production, utilize an alkali metal chlorate salt, a reducing agent, and an acid in a chemical process for producing chlorine dioxide. These generators and the processes employed also produce by-product salts such as sodium chloride, sodium sulfate, or sodium bisulfate. In pulp and paper mills, the typical by-product is sodium sulfate (saltcake) which is converted into a sulfur salt of sodium in a high temperature boiler and used in the paper process. Boilers require energy and the paper mills have a limited boiler capacity. Increasing the production of chlorine dioxide generally means increased capital investment to provide the added boiler capacity required to process the added amounts of saltcake by-product produced.
Thus a process which reduces the amount of a by-product salt, such as sodium chloride or sodium sulfate, produced while efficiently generating chlorine dioxide is commercially desireable.
U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5.degree. to 40.degree. C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO.sub.3. This process requires the regeneration of the cationic exchange resin with acid to remove the alkali metal ions and the treatment or disposal of the acidic salt solution.
K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolyzed solution contains a mixture of chlorine dioxide and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride formed.
In U.S. Pat. No. 4,683,039, Twardowski et al. describe a method for producing chlorine dioxide in which the chlorine dioxide is produced in a generator by the reaction of sodium chlorate and hydrochloric acid. After separating chlorine dioxide gas, the remaining sodium chloride solution is fed to a three-compartment cell to form sodium hydroxide and an acidified liquor which is returned to the chlorine dioxide generator.
Each of the above processes produces a fixed amount and type of by-product salt.
M. Lipsztajn et al. teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. Chlorate ions are transferred through an anion-exchange membrane and sodium ions are passed through a cation-exchange membrane (U.S. Pat. No. 4,915,927, Apr. 10, 1990).